Radar apparatus measure at least one of a distance between a target and a measuring place and a direction of the target as viewed from the measuring place by radiating radio waves to the space from the measuring place on the basis of a pulse signal and receiving a pulse signal of reflection waves reflected by the target. In recent years, radar apparatus which can detect targets including automobiles and pedestrians by a high-resolution measurement using short-wavelength radio waves such as microwaves or millimeter waves have been being developed.
Radar apparatus receive a signal that is a mixture of reflection waves coming from a nearby target and reflection waves coming from a distant target. In particular, range sidelobes occur due to a signal of reflection waves coming from a nearby target. Where range sidelobes and a main lobe of a signal of reflection waves coming from a distant target exist in mixture, the accuracy of detection of the distant target by a radar apparatus may be lowered.
Therefore, radar apparatus which use a pulse signal and are required to perform high-resolution measurement on plural targets are required to transmit a pulse wave or a pulse-modulated wave having an autocorrelation characteristic with low range sidelobe levels (hereinafter referred to as low range sidelobe characteristic).
When an automobile and a pedestrian are located at the same distance from a measuring place, a radar apparatus receives a signal that is a mixture of signals of reflection waves coming from the automobile and the pedestrian which have different radar cross sections (RCSs). This is because in general the radar cross section of a pedestrian is smaller than that of an automobile.
Therefore, radar apparatus are required to properly receive reflection wave signals coming from an automobile and a pedestrian even if they are located at the same distance from a measuring place. Since the output level (reception level) of a reflection wave signal varies depending on the distance or type of a target, radar apparatus are required to have a reception dynamic range that enables reception of reflection wave signals of various reception levels.
Where a high-frequency signal such as a millimeter wave signal is used in conventional radar apparatus that exhibit a low range sidelobe characteristic, circuit errors (e.g., phase errors) are mixed into an RF (radio frequency) circuit and an analog baseband circuit that are provided in a radar transmitter and a radar receiver. As a result, such radar apparatus are lowered in target ranging performance than ones having an ideal characteristic that is free of circuit errors.
In connection with the above problem, Patent document 1, for example, discloses an A/D conversion circuit in which a highpass filter is disposed downstream of an AD converter and a DC offset is thereby eliminated. Furthermore, Patent document 2 discloses a radar apparatus in which a bandpass filter is provided in the input stage of an AD converter and a DC offset is thereby eliminated. Non-patent document 2 discloses a circuit configuration for correction of IQ imbalance circuit errors.